Prostate cancer (CaP) is the most common malignancy in American men and second leading cause of cancer mortality (1). Serum-prostate specific antigen (PSA) tests have revolutionized the early detection of CaP (2). Although PSA has revolutionized early detection of prostate cancer, there is still a very high false positive rate. The increasing incidence of CaP has translated into wider use of radical prostatectomy as well as other therapies for localized disease (3-5). The wide spectrum of biologic behavior (6) exhibited by prostatic neoplasms poses a difficult problem in predicting the clinical course for the individual patient (3-5). Traditional prognostic markers such as grade, clinical stage, and pretreatment PSA have limited prognostic value for individual men (3-5). A more reliable technique for the evaluation and prognosis of CaP is desirable.
Molecular studies have shown a significant heterogeneity between multiple cancer foci present in a cancerous prostate gland (7,8). These studies have also documented that the metastatic lesion can arise from cancer foci other than those present in dominant tumors (7). Approximately 50-60% of patients treated with radical prostatectomy for localized prostate carcinomas are found to have microscopic disease that is not organ-confined, and a significant portion of these patients relapse (9). Therefore, identification and characterization of genetic alterations defining CaP onset and progression is crucial in understanding the biology and clinical course of the disease.
Despite recent intensive research investigations, much remains to be learned about specific molecular defects associated with CaP onset and progression (6, 10-15). Alterations of the tumor suppressor gene p53, bcl-2 and the androgen receptor (AR), are frequently reported in advanced CaP (6, 10-15). However, the exact role of these genetic defects in the genesis and progression of CaP is poorly understood (6, 10-15). Recent studies have shown that the “focal p53 immunostaining” or bcl-2 immunostaining in radical prostatectomy specimens were independent prognostic markers for cancer recurrence after surgery (16-19). Furthermore, the combination of p53 and bcl-2 alterations was a stronger predictor of cancer recurrence after radical prostatectomy (18).
The roles of several new chromosome loci harboring putative proto-oncogenes or tumor suppressor genes are being currently evaluated in CaP (7-13). High frequency of allelic losses on 8p21-22, 7q31.1, 10q23-25 and 16q24 loci have been shown in CaP (6, 10-15). PTEN1/MMAC1, a recently discovered tumor suppressor gene on chromosome 10q25, is frequently altered in advanced CaP (20, 21). Gains of chromosome 8q24 harboring c-myc and prostate stem-cell antigen (PSCA) genes have also been shown in prostate cancer (22, 23). Studies utilizing comparative genomic hybridization (CGH) have shown frequent losses of novel chromosomal loci including 2q, 5q and 6q and gains of 11p, 12q, 3q, 4q and 2p in CaP (24, 25). The inventors have recently mapped a 1.5 megabase interval at 6q16-21 which may contain the putative tumor suppressor gene involved in a subset of prostate tumors. The risk for 6q LOH to non-organ confined disease was five fold higher than for organ confined disease (26). Chromosome regions, 1q24-25 and Xq27-28 have been linked to familial CaP (27, 28).
It is evident that multiple molecular approaches need to be explored to identify CaP- associated genetic alterations. Emerging strategies for defining cancer specific genetic alterations and characterizing androgen regulated genes in rat prostate and LNCaP human prostate cancer cell models include, among others, the study of global gene expression profiles in cancer cells and corresponding normal cells by differential display (DD) (29) and more recent techniques, such as serial amplification of gene expression (SAGE) (30) and DNA micro-arrays (31; U.S. Pat. Nos. 5,744,305 and 5,837,832 which are herein incorporated by reference) followed by targeted analyses of promising candidates. Our laboratory has also employed DD, SAGE and DNA microarrays to study CaP associated gene expression alterations (32-33). Each of these techniques, however, is limited. The number of transcripts that can be analyzed is the major limitation encountered in subtractive hybridization and differential display approaches. Furthermore, while cDNA microarray approaches can determine expression of a large number of genes in a high throughput manner, the current limitations of cDNA arrays include the presence of specific arrays used for analyses and the inability to discover novel genes.
While alterations of critical tumor-suppressor genes and oncogenes are important in prostate tumorogenesis, it is also recognized that hormonal mechanisms play equally important roles in prostate tumorogenesis. The cornerstone of therapy in patients with metastatic disease is androgen ablation, commonly referred to as “hormonal therapy (34),” which is dependent on the inhibition of androgen signaling in prostate cancer cells. Androgen ablation can be achieved, for example, by orchiectomy, by the administration of estrogen, or more recently by one of the luteinizing hormone-releasing hormone agonists. Recent clinical trials have demonstrated the efficacy of combining an antiandrogen to orchiectomy or a luteinizing hormone-releasing hormone to block the remaining androgens produced by the adrenal glands. Although approximately 80% of patients initially respond to hormonal ablation, the vast majority of patients eventually relapse (35), presumably due to neoplastic clones of cells which become refractory to this therapy.
Alterations of the androgen receptor gene by mutations in the hormone binding domain of the AR or by amplification of the AR gene have been reported in advanced stages of CaP. Much remains to be learned, however, about the molecular mechanisms of the AR-mediated cell signaling in prostate growth and tumorogenesis (36-43). Our earlier studies have also described mutations of the AR in a subset of CaP (40). Mutations of the AR are reported to modify the ligand (androgen) binding of the AR by making the receptor promiscuous, so that it may bind to estrogen, progesterone, and related molecules, in addition to the androgens (36,38,42). Altered ligand binding specificity of the mutant AR may provide one of the mechanisms for increased function in cancer cells. Amplifications of the AR gene in hormone-refractory CaP represent yet another scenario where increase in AR function is associated with tumor progression (44,45).
Several growth factors commonly involved in cell proliferation and tumorogenesis, e.g., IGF1, EGF, and others, have been shown to activate the transcription transactivation functions of the AR (46). The co-activator of the AR transcription factor functions may also play a role in prostate cancer (47). Recent studies analyzing expression of the androgen-regulated genes (ARGs) in hormone sensitive and refractory CWR22 nude mice xenograft models (48) have also shown expression of several androgen regulated genes in AR positive recurrent tumors following castration, suggesting activation of AR in these tumors (49).
In addition to the alterations of the androgen signaling pathway(s) in prostate tumor progression, androgen mechanisms are suspected to play a role in the predisposition to CaP. Prolonged administration of high levels of testosterone has been shown to induce CaP in rats (50-52). Although recent evidence suggests an association of androgen levels and risk of CaP, this specific observation remains to be established. (53). An independent line of investigations addressing the length of inherited polyglutamine (CAG) repeat sequence in the AR gene and CaP risk have shown that men with shorter repeats were at high risk of distant metastasis and fatal CaP (54,55). Moreover, the size distribution of AR CAG repeats in various ethnic groups has also suggested a possible relationship of shorter CAG repeats and increased prostate cancer risks in African-American men (56,57). Biochemical experiments evaluating AR-CAG repeat length and in vitro transcription transactivation functions of the AR revealed that AR with shorter CAG repeats possessed a more potent transcription trans-activation activity (58). Thus, molecular epidemiologic studies and biochemical experimentation suggest that gain of AR function, consequently resulting in transcriptional transactivation of downstream targets of the AR gene, may play an important role in CaP initiation. However, downstream targets of AR must be defined in order to understand the biologic basis of these observations.
The biologic effects of androgen on target cells, e.g., prostatic epithelial cell proliferation and differentiation as well as the androgen ablation-induced cell death, are likely mediated by transcriptional regulation of ARGs by the androgen receptor (reviewed in 59). Abrogation of androgen signaling resulting from structural changes in the androgen gene or functional alterations of AR due to modulation of AR functions by other proteins would have profound effects on transcriptional regulation of genes regulated by AR and, thus, on the growth and development of the prostate gland, including abnormal growth characterized by benign prostatic hyperplasia and prostatic cancer. The nature of ARGs in the context of CaP initiation and progression, however, remains largely unknown. Since forced proliferation of the AR prostate cancer cells lacking AR induces cell-death related phenotypes (60), the studies utilizing AR expression via heterologous promoters in cell cultures have failed to address the observations relating to gain of AR functions and prostate cancer progression. Moreover, suitable animal models to assess gain of AR functions do not exist. Therefore, the expression profile of androgen responsive genes (ARGs) has potential to serve as read-out of the AR signaling status. Such a read-out may also define potential biomarkers for onset and progression of those prostate cancers which may involve abrogation of the androgen signaling pathway. Furthermore, functional analysis of androgen regulated genes will help understand the biochemical components of the androgen signaling pathways.